In recent years the requirements that silver halide emulsions for photographic materials are expected to satisfy have been getting severer. Such emulsions are required to be more and more upgraded in photographic performance, the demand being intensified for higher sensitivity, better graininess, enhanced sharpness, lower fogging density, and sufficiently high optical density.
Well known as high sensitivity emulsions in an attempt to meet these requirements are silver bromoiodide emulsions containing less than 10 mol % silver iodide. And known as conventional methods for preparing these emulsions are so-called an ammonia method, a neutralization method, an acidity method, and the like, where the conditions of pH and pAg are controlled, and a single jet method and a double jet method for mixing.
On the basis of these known techniques, there have been detailed studies on technical potentialities with an object of upgrading sensitivity, graininess, sharpness and reducing fogging, some of the accomplishments of such studies having been brought into practice. Especially, the emulsions of silver bromide and silver bromoiodide have been studied to such an extent that not only crystal phase, grain size distribution, etc. of an emulsion but density distribution of silver iodide in the individual silver halide grains has come to be controlled in some of the studies.
In an attempt to upgrade a silver halide emulsion in photographic performance such as sensitivity, graininess, sharpness, fogging density and covering power, the most orthodox way is to enhance the quantum efficiency of silver halide. For this purpose a related scope of knowledge in solid-state physics is positively being introduced into the studies. According to observations on quantum efficiency based on theoretical calculations, an effective method for enhancing the quantum efficiency is to prepare an emulsion in a monodispersed system by narrowing grain size distribution. Moreover in chemical sensitization, a process to sensitize silver halide emulsions, a monodispersed emulsion is considered advantageous in an attempt to achieve an high sensitivity efficiently while fogging is held low.
What is required for commercial production of a monodispersed emulsion is to control the rates at which a silver ion and halide ion, both theoretically determined, are fed to a reaction system and to give adequate stirring under strict control of pAg and pH as specified in Japanese Patent Publication Open to Public Inspection (Toku Kai Sho) (hereinafter referred to as "Japanese Patent O.P.I. Publication") No. 54-48521/1979. The silver halide emulsion obtainable at these conditions consists of so-called regular crystals in either form of cube, octahedron or tetradecahedron with (100) and (111) planes in various ratios. It is known that by forming such regular crystal grains it is possible to efficiently sensitize silver halide emulsions.
In an attempt to impart high sensitivity to silver halide grains a success is reported by Japanese Patent O.P.I. Publication Nos. 61-35440/1986 and 60-222842/1985, both of which disclose silver bromoiodide grains with (110) planes, having the excellent photographic properties, whereas an accomplishment in reducing fog is reported by Japanese Patent Examined Publication (Toku Koh Sho) No. 55-42737/1980, which discloses a photographic emulsion containing silver chlorobromide grains in the form of rhombic dodecahedron with (110) planes.
Further augmentation of sensitivity is reported to be possible in Japanese Patent O.P.I. Publication No. 61-83531/1986, according to which silver bromide and silver bromoiodide crystals produced have a ridgeline in the middle of (110) plane. This crystal face is considered to be of a very high order, the relevant properties being described in the disclosure of Japanese Patent O.P.I. Publication No. 61-83531/1986. This crystal face is represented as (nnl), for example, (331).
There are described crystal faces other than the above in Japanese Patent O.P.I. Publication Nos. 62-124551/1987, 62-124550/1987 and 62-123447/1987.
On the other hand, some silver bromoiodide emulsions consisting of polydispersed twinned crystals are known to be advantageous for production of high sensitivity photographic films. Flat twinned crystals are included in some of such silver bromoiodide emulsions, as is described in, for example, Japanese Patent O.P.I. Publication No. 58-113927/1983.
The application of such twinned crystals is effective in enhancing sensitivity, but the crystals tend to become such irregular in shape and size that it is difficult to control the photographic properties accurately and to achieve good reproducibility.
Consideration of the effect of chemical sensitization raises as a problem a disadvantage or difficulty of commercial production of silver halide grains having (111) planes, since chemical sensitization of regular crystals is so dependent upon a crystal phase that, for example, compared with (100) planes a sensitizing reaction by an ordinary method produces a large number of sulfur sensitization nuclei on (111) planes, and a scattered formation of latent images results eventually in inefficient sensitization.
For example, both Japanese Patent O.P.I. Publication No. 50-63914/1975 and West German OLS Patent No. 2,419,798 describe an augmentation of sensitivity, where a monodispersed silver halide emulsion consisting of cubes containing silver bromide in a molar ratio of over 80% is sulfur sensitized and then, a hydroxytetrazaindene compound is added, whereas these Publications disclose that sensitivity of an emulsion containing the crystals other than cube, for example, octahedron consisting substantially of (111) planes, rather decreases or very slightly increases.
Although the regular crystals mentioned above lend themselves to accurate control for giving the grains specific character, the presence of so many equivalent faces, edges and corners in an isotropic regular crystal results in scattering equivalently the effects of chemical sensitization and exposure so that possibility of activation of the light-sensitive nuclei and/or the image-developing nuclei, which potentialize development, is reduced. The application of the regular crystals runs counter to the so-called principle of concentration. In other words, the effects of chemical sensitization and exposure are not concentrated because of scattering of the active nuclei mentioned above.
As can be seen from the above, there is a close relationship between the crystal faces of silver halide grains and the photographic properties, and further investigation of such relationship leads to possibility of successful development of silver halide emulsions upgraded in photographic properties.